This invention relates to a second harmonic wave generating device (hereinafter referred to as "SHG device") of a thin film waveguide structure with a high conversion efficiency.
A SHG device utilizes nonlinear optical effects of a nonlinear optical crystal material to convert wavelength .lambda. of incident laser light to wavelength 1/2.lambda., which is outputted. Since the output light has a half wavelength of incident light, the device can be used in an optical disc memory and CD player to achieve a 4-times increase in recording density, and can be used in a laser printer and photolithography with enhanced resolution.
Heretofore, a bulk single crystal of a nonlinear optical material using a high-output-power gas laser as a light source has been used as a SHG device. However, with recent increases in demand for compact optical disc systems and laser printers and since gas laser requires an external modulator for optical modulation and is not suited for compact design, a SHG device that enables use of a semiconductor laser, which can be directly modulated and is lower in cost and easier to handle than gas laser, has been in demand.
When a semiconductor laser is used as a light source, since the semiconductor laser has a low output power of several mW to several ten mW, a SHG device of a thin film waveguide structure which has a particularly high conversion efficiency has been required.
Generation of second harmonic optical wave using a thin film waveguide has advantages that: (1) energy of light concentrated on the thin film can be utilized, (2) since optical wave is confined within the thin film and does not diffuse, interaction is possible over a long distance, and (3) a substance, which cannot make phase matching in the bulk state, becomes able to make phase matching by utilizing mode dispersion of thin film (Miyama and Miyazaki; Technical Report of the Electronic Communication Society, OQE75-6 (1975), Miyazaki, Hoshino, and Akao; Proceedings of Electromagnetic Field Theory Research Conference, EMT-78-5 (1978)).
However, in order to obtain a SHG device of a thin film waveguide structure, it has heretofore been necessary to conduct experiments with substrates of different materials and thin film waveguide layers of different materials and thicknesses at an objective fundamental wavelength to find conditions for generation of a second harmonic wave and to determine the structure, thus requiring very inefficient work.
The inventors have conducted intensive studies and have found that a second harmonic wave can be generated very efficiently by satisfying a specific relation of a fundamental wavelength (.lambda. .mu.m), a thickness (T .mu.m) of the thin film waveguide layer, an ordinary refractive index (n.sub.OS1) of the substrate at the fundamental wavelength (.lambda. .mu.m), an ordinary refractive index (n.sub.OF1) of the thin film waveguide layer at the fundamental wavelength (.lambda. .mu.m), an extraordinary refractive index (n.sub.eS2) of the substrate at a second harmonic wavelength (.lambda. .mu.m/2), and an extraordinary refractive index (n.sub.eF2) of the thin film waveguide layer at the second harmonic wavelength (.lambda. .mu.m/2), thus accomplishing the present invention.
Heretofore, in the fabrication of a SHG device, the thickness of the thin film waveguide has been adjusted by optically polishing the film. Through intensive studies the inventors have found that a dry etching technique is most suitable as a processing method which is more simple than dry etching and enables a low surface roughness and a uniform film thickness.